The present invention relates to optical heads for optically recording, reproducing and erasing information on and from an information recording medium such as an optical disc, and more particularly to such an optical head equipped with a tracking error detecting system to accurately keeping the position of an objective lens of the optical head with respect to a track on the information recording medium.
Various types of tracking error detecting systems for optical heads have been proposed hitherto. One known arrangement will briefly be described hereinbelow with reference to FIGS. 1 and 2A to 2C. In FIG. 1, a light beam emitted from a light source 1 is first reflected by a half-mirror (beam splitter) 2a and then focused on a disc 4 through an objective lens 3. On a surface of the disc 4 is formed an information track, not shown, which is constructed as a spirally continuous groove. The reflection light beam from the disc 4 passes through the same half-mirror 2a before being reflected by another half-mirror 2b so as to be incident on a detecting means 15 for focus error signal detection and information signal detection. The detecting means 15 is coupled to an actuator 3a whereby the objective lens 3 is moved in directions Y in accordance with the focus error signal detection for the focus servo operation. On the other hand, the light beam passing through the half-mirror 2b reaches a two-divided photodetector 5 to form a light spot 7 thereon. The two-division photodetector 5 having thereon a division line 5a to divide the light-detecting region into two light-detecting subregions, the output electric signals of which are fed to a differential amplifier 6 so as to detect the difference between the output signals to obtain a tracking error signal. This technique is so-called Push-pull method and widely used in the optical recording and reproducing art. The tracking error signal is supplied through a servo amplifier 16 to the actuator 3a whereby the objective lens 3 is moved in the positive (+) and negative (-) directions X for tracking servo operation.
Although satisfactory for tracking error signal detection with a simple structure, the above-described tracking error detecting system has the following disadvantages.
FIGS. 2A to 2C are illustrations for describing the tracking error signals in connection with the position of the light spot 7 on the two-division photodetector 5. As illustrated in FIG. 2A, the light spot 2 is formed thereon so as to be symmetrical relative to the division line 5a in cases where the objective lens 3 takes a neutral position in the direction X. If the target voltage in the tracking servo actuating condition is zero, the tracking error signal obtained when the optical disc 4 is rotating and only the focus servo is actuated symmetrically varies with respect to the zero voltage in accordance with the radial runout of the information track on the recording medium 4. FIG. 2B shows the case that the position of the light spot 7 is changed in response to the objective lens 3 being moved in the positive direction X (to the right side). In this case, the light spot 7 on the two-division photodetector 5 is shifted so as not to be symmetrical with respect to the division line 5a. Thus, the tracking error signal obtainable does not vary symmetrically relative to the zero voltage but is superimposed with an offset voltage +V. On the other hand, in the case illustrated in FIG. 2C, the light spot 7 is shifted to the left side in response to the objective lens 3 being moved in the negative direction X, whereby the tracking error signal obtainable is superimposed with an offset voltage -V.
In FIG. 2B, if two times of the amplitude of the tracking error signal is taken to be S, when the value V/S exceeds 10%, the control performance of the tracking servo generally deteriorates. When the moving amount of the objective lens 3 in the directions X reaches about 50 .mu.m, in the case of the conventional push-pull system, the value V/S exceeds 10%.
In the case of performing a high-speed search of the target track, it is general that the tracking servo enters into the actuating state after the start of the high-speed operation of the optical head. Thus, the objective lens 3 is shifted from the nuetral position in the directions X due to the acceleration or the like occuring in response to the movement of the optical head and the light spot 7 results in the condition as illustrated in FIG. 2B or 2C. Under these conditions, the tracking servo tends to enter into an undesirable actuating state. Even if the tracking servo fortunately enters into a desirable state, the follow-up performance thereof extremely deteriorates.
One possible solution for the above-described problems is disclosed in the Japanese Patent Provisional Publication No. 60-214429, the technique of which involves including a photodetector whose light-detecting region is divided into four or more sub-regions so as to obtain a tracking error signal on the basis of the output signals of the four or more light-detecting sub-regions. Although according to this system the offset of the tracking error signal produced due to the movement of the objective lens in the tracking direction can be reduced, there is a problem in that a great number of amplifiers are required for the processing.